LM4839LQ NSC [National Semiconductor], LM4839LQ Datasheet - Page 14

• Manufacturer Part Number: LM4839LQ
• Description: Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
EXPOSED-DAP MOUNTING CONSIDERATIONS
The LM4839’s exposed-DAP (die attach paddle) packages
(MTE, LQ) provide a low thermal resistance between the die
and the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane and, finally, surrounding
air. The result is a low voltage audio power amplifier that
produces 2.1W at 1% THD with a 4 load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4839’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MTE and LQ packages must have their exposed DAPs
soldered to a grounded copper pad on the PCB. The DAP’s
PCB copper pad is connected to a large plane of continuous
unbroken copper. This plane forms a thermal mass and heat
sink and radiation area. Place the heat sink area on either
outside plane in the case of a two-sided PCB, or on an inner
layer of a board with more than two layers. Connect the DAP
copper pad to the inner layer or backside copper heat sink
area with 32(4x8) (MTE) or 6(3x2) (LQ) vias. The via diam-
eter should be 0.012in–0.013in with a 1.27mm pitch. Ensure
efficient thermal conductivity by plating-through and solder-
filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
necessary for 5V operation with a 4
not placed on the same PCB layer as the LM4839 should be
5in
The last two area recommendations apply for 25˚C ambient
temperature. Increase the area to compensate for ambient
temperatures above 25˚C. In systems using cooling fans, the
LM4839MTE can take advantage of forced air cooling. With
an air flow rate of 450 linear-feet per minute and a 2.5in
exposed copper or 5.0in
the LM4839MTE can continuously drive a 3
power. The LM4839LQ achieves the same output power
level without forced air cooling. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4839’s thermal shutdown
protection. The LM4839’s power de-rating curve in the Typi-
cal Performance Characteristics shows the maximum
power dissipation versus temperature. Example PCB layouts
for the exposed-DAP TSSOP and LQ packages are shown in
the Demonstration Board Layout section. Further detailed
and specific information concerning PCB layout, fabrication,
and mounting an LQ (LLP) package is available in National
Semiconductor’s AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1 trace resistance reduces
the output power dissipated by a 4 load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
2
(min) for the same supply voltage and load resistance.
2
inner layer copper plane heatsink,
load. Heatsink areas
AND 4
2
(min) area is
load to full
LOADS
2
14
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1 , the LM4839 output stage consists of
two pairs of operational amplifiers, forming a two-channel
(channel A and channel B) stereo amplifier. (Though the
following discusses channel A, it applies equally to channel
B.)
Figure 1 shows that the first amplifier’s negative (-) output
serves as the second amplifier’s input. This results in both
amplifiers producing signals identical in magnitude, but 180˚
out of phase. Taking advantage of this phase difference, a
load is placed between −OUTA and +OUTA and driven dif-
ferentially (commonly referred to as “bridge mode”). This
results in a differential gain of
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage
swing across the load. This produces four times the output
power when compared to a single-ended amplifier under the
same conditions. This increase in attainable output power
assumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
P
DMAX
= (V
A
DD
VD
)
2
= 2
/(2
2
*
R
(R
L
) Single-Ended
f
/R
i
)
(1)
(2)